The present invention relates to portable radio equipment having a liquid crystal display (LCD) or similar display which itself does not emit light and, more particularly, to portable radio equipment having a function of back-lighting a display thereof by an electroluminescent (EL) lamp.
With portable radio equipment of the type described, it has been customary to back-light a display by the following implementation. Specifically, a phototransistor or similar light-sensitive element is so disposed as to sense light, or illumination, around the display, while a light-sensitive circuit is connected to the light-sensitive element. An output of the light-sensitive circuit representative of a dark condition and an output indicating that the display is in operation are ANDed. The resulting AND output operates a driver associated with a light source so as to energize a light emitting element which is independent of the light-sensitive element, whereby the display is back-lighted. A problem with this kind of prior art equipment is that the light-sensitive element responsive to illumination is simply located relatively close to the display to sense illumination around the display. Specifically, the light-sensitive element cannot sense the illumination of light incident to the display directly. Hence, when spot light is incident to the light-sensitive element only and not to the display, the display is not back-lighted at all despite that it is too dark to see. Conversely, when spot light illuminates only the display, the display is back-lighted despite that it is readable without back-light. Such wasteful consumption of power has to be eliminated because this type of radio equipment is powered by a miniature battery whose capacity is small. Another problem is that the equipment needs a casing which has an exclusive opening for the light-sensitive element, in addition to an opening which is associated with the display.